High-brightness, low-emittance, and high-coherence X-rays in various wavelength regions from soft X-rays to hard X-rays have become available at third-generation synchrotron radiation facilities represented by SPring-8. This has dramatically improved analytical sensitivities and spatial resolutions at various analyses such as fluorescent X-ray analysis, photoelectron spectrometry, and X-ray diffraction. These X-ray analyses and X-ray microscopic approaches using radiation light not only provide high sensitivities and high resolutions but also allow nondestructive observations, and thus are currently being employed in the fields of medicine, biology, and material science, and the like.
Highly collected X-ray nanobeams are required to utilize various X-ray analytical technologies with high spatial resolutions at synchrotron radiation facilities. A group of the inventors has already succeeded in collecting an X-ray with a wavelength of 0.6 Å in a spot diameter of 100 nm or less, by using a light collection optical system including a Kirkpatrick and Baez (K-B) mirror. This success is largely due to a uniquely developed high-precision mirror processing technique and high-precision mirror shape measurement techniques. This processing technique refers to numerically-controlled elastic emission machining (EEM) which is performed on a process principle: a high shear flow of ultrapure water mixed with fine particles is formed along a surface of a mirror to be processed; the fine particles combine together with atoms on the surface of the mirror by a kind of chemical reaction; and the surface atoms are removed with movement of the fine particles. In addition, the shape measurement technologies refer to microstitching interferometry (MSI) and relative angle determinable stitching interferometry (RADSI) which are performed on a measurement principle that pieces of partial shape data from an interferometer capable of high-precision shape measurement of small areas are put together to obtain the entire shape data. Using the shape measurement techniques makes it possible to measure accurately the shape of an X-ray mirror in all space wavelength ranges with a measurement reproducibility of 1 nm or less of PV value. The group has successfully prepared an X-ray light collecting mirror with an accuracy of 2 nm (PV value) using these techniques, thereby to realize diffraction-limited light collection of SPring-8 hard X-rays at a level of sub-30 nm.
The inventors aim to realize sub-10 nm light collection for implementation of the world's best ultrahigh-resolution scanning X-ray microscope and ultrahigh-resolution X-ray micro CT. To that end, extremely strict shape accuracy is required for X-ray mirrors as follows: a shape error is P-V1 nm or less in mid- and long-term space wavelengths; a designed mirror shape has a deep curve; a multilayer film is essentially formed on a mirror surface to provide a deep X-ray incident angle, and the like. Accordingly, it is extremely difficult to determine a phase error in a surface of an X-ray mirror with respect to an ideal surface by off-line measurement using an interferometer or the like. The inventors therefore have proposed an at-wavelength metrology in which a phase error in a mirror surface is determined by phase retrieval calculation only from X-ray intensity profile information in a light collection plane, and proposed an X-ray collection method in which a phase error of a light collection optical system is corrected using the foregoing metrology to eliminate irregularities in a wavefront of a focal plane (JP 2006-357566 (JP 2008-164553 A)). To calculate precisely a phase error of an X-ray mirror by the phase retrieval method, it is essentially required to acquire an accurate X-ray collection intensity profile.
Conventionally, an X-ray beam intensity profile is measured in such a manner as to cut off an X-ray beam little by little by a knife edge or a wire while measuring changes in light intensity as described in Patent Document 1. FIG. 14 shows a measurement optical system using a wire scanning method. In this optical system, an incident X-ray 100 is passed through a slit 101 so as to be limited to a predetermined width, then is passed through an ion chamber 102, and then is reflected and collected by a surface of an X-ray mirror 103. In the foregoing arrangement, an Au wire 104 with a diameter of 200 μm sufficiently larger than a diameter of an X-ray beam is run by a piezo stage in a light collection plane vertically to the mirror surface, thereby to gradually cut off a collected beam while measuring changes in X-ray intensity behind the focal point through the slit 105 by an X-ray detector 106. In this arrangement, as the X-ray detector 106, an avalanche photodiode (APD) with high sensitivity and fast output responsibility is used. The X-ray intensities measured by the X-ray detector 106 are standardized in accordance with an incident X-ray intensity measured at the ion chamber 102. The slit 105 is provided to eliminate influence of inclination of the wire 104 with respect to the beam on measurement of a light collection intensity profile. FIG. 15(a) shows changes in X-ray intensity profile measured by the X-ray detector 106. These changes are differentiated with respect to wire positions, thereby to obtain a light collection intensity profile as shown in FIG. 15(b).
However, the wire scanning method has two problems: it is difficult to prepare a geometrically sharp knife edge with a sufficient thickness so as not to let an X-ray pass through; and noise generated at intensity measurement is enhanced at the time of differentiation. In addition, although accurate information is needed in a wide base region of an X-ray intensity profile to calculate precisely a phase error of an X-ray mirror by phase retrieval, the conventional wire scanning method provides information in this region with low reliability.
Accordingly, in order to provide a method and apparatus for precise measurement of an X-ray nanobeam intensity distribution that overcome the problem of noise enhancement due to background noise and differentiation associated with the wire scanning method and realize higher-precision X-ray beam profile measurement, the inventors propose a method for precise measurement of an X-ray nanobeam intensity distribution that use a dark-field metrology to run a knife edge so as to cut across an X-ray beam and measure an X-ray intensity by an X-ray detector disposed behind the knife edge at a position geometrically dark with respect to an X-ray source, thereby to measure an X-ray intensity distribution in a cross section of the X-ray beam, wherein the knife edge is made of a heavy metal with the effect of advancing a phase of an X-ray passing through the knife edge, a thickness of the knife edge is set so as to obtain a phase shift to an extent that the transmission X-ray and a diffraction X-ray diffracted by a leading end of the knife edge reinforce each other, and an X-ray formed by overlapping of the diffraction X-ray and the transmission X-ray is measured by the X-ray detector.
Patent Document 1: JP-A No. 10-319196